The present invention relates to semiconductor technology, and more particularly to a CMOS image sensor, a method for manufacturing the same, and an electronic device including a CMOS image sensor.
An image sensor is a semiconductor device that is capable to convert an optical image into an electric signal. There are two main types of image sensors such as charge-coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) image sensors. CCD image sensors have the advantages of higher image sensitivity and low noise, but CCD image sensors are difficult to be integrated with other devices and have high power consumption for large arrays. In contrast, CMOS image sensors have a simple manufacture process and can be easily integrated with other devices to achieve a higher level of integration. Furthermore, CMOS image sensors have a smaller size, lightweight, lower power consumption, and low cost. CMOS image sensors are currently widely used in digital still cameras, digital video cameras, camera phones, medical imaging devices (e.g., gastroscopy), rear viewing cameras and monitor systems for motor vehicles.
A conventional CMOS image sensor includes a semiconductor substrate, multiple pixels arranged in an array configuration in the semiconductor substrate, and shallow trench isolations (STI) disposed between adjacent pixels. FIG. 1 is simplified cross-sectional view of a conventional backside illuminated CMOS image sensor. The CMOS image sensor includes a semiconductor substrate 100, multiple pixels (two pixels 11 and 12 are shown), and multiple shallow trench isolations (STI) disposed between two adjacent pixels (a STI 101 is shown between pixels 11 and 12). The CMOS image sensor further includes a semiconductor layer comprising multiple active elements (transistors) and passive elements such as resistors, capacitors, and others. The semiconductor layer is shown as a polysilicon layer 1500. The CMOS image sensor also includes transistors and photodiodes for converting photons into electrical signals, which are amplified by the transistors and provided to other electronic devices for processing. The shallow trench isolations are used to isolate adjacent pixels. Incident light from the back surface of the semiconductor substrate reaches the photodiode of pixel 12 (indicted as an arrow), the photodiode captures incident photons and converts them into photoelectrons (indicated as “e−”), which are collected in the n+ layer of the photodiode. Since incident light is entered from the back surface of the semiconductor substrate to the pixels, this type of CMOS image sensors is generally referred to as backside-illuminated CMOS image sensors.
In the prior art, a backside-illuminated CMOS image sensor has two sources of crosstalk: one crosstalk source is optical crosstalk, and the other crosstalk source is electrical crosstalk. Optical crosstalk is mainly caused by incident light having large incident angles θ into adjacent photodiodes, as shown in FIG. 2. Electrical crosstalk is mainly caused by photoelectrons 301 wandering into adjacent photodiodes, as shown in FIG. 3. Both sources of crosstalk can affect the image quality of a CMOS image sensor, namely, the greater the crosstalk, the worse the quality of the final image.
Therefore, a new solution is needed to reduce crosstalk in the CMOS image sensors.